Many applications require controlled movement between two elements. For example, hinges are often used for providing a movable connection between different portions of an apparatus. This movement often requires repeated cycles of smooth and substantially repeatable force exerted by the user of the device to position one element relative to another element. The result of this movement is positioning of the device in a way that resists further movement against disturbing environmental forces such as imposed by gravity or vibration.
One such example of such a hinge mechanism is in conjunction with a laptop computer, which hinges a screen relative to a base. Other applications may include relative linear motion between two elements. For example, it may be desired to control the up and down motion of a headrest relative to a seat.
Unlike simpler bearing applications, whose object is to minimize wear by minimizing friction, controlled movement and positioning devices must generate significant and substantially repeatable forces or torques through controlled friction, often over many thousands of operating cycles. Furthermore, the need to minimize the size of these elements relative to these forces and torques results in the generation of internal stresses hundreds of times greater than those experienced by typical bearing applications.
This need to withstand unusually large stresses has limited the usable material choices for these applications. Typical lower strength bearing materials are not only unpredictable at these stress conditions given the lack of published performance data, but are also not suitable given the multiple failure modes which may appear at these conditions.
For these reasons, the preferred technology set for such devices has been precision-formed hardened steel surfaces lubricated with grease. However, this technology results in higher than desired cost, complexity of manufacture, and complications attendant to grease application.
Engineering plastics, while finding wide use in bearing applications, have found only limited use in such controlled movement and positioning applications. While eliminating the need for grease with their self-lubricating properties, these engineering plastics typically cannot withstand high internal stresses without stress relaxation, creep, wear resulting in torque loss, or torque increases resulting in catastrophic failure of the mechanism, making them unsuitable for applications requiring both small size and large repeatable forces and torques. A few engineering plastics such as UHMW-PE have partially overcome this size drawback, but require more expensive manufacturing operations than molded or extruded plastics to achieve a finished, functional shape.
For these and other reasons, there exists a need for the present invention.